Conventionally, the mainstream construction machine such as a hydraulic excavator includes a hydraulic actuator that drives a hydraulic pump with an engine so as to drive and operate a working device using the power of this hydraulic pump. However, nowadays, for example, to improve the fuel efficiency of the engine, reduce the noise level, and reduce the exhaust gas amount, a hybrid-type construction machine is developed and put into practical use. The hybrid-type construction machine includes a generator-motor, which can generate electric power by the engine and is used for assisting the hydraulic pump, and an electric storage device (a capacitor, a battery), which supplies electric power to drive the generator-motor and charges the electric power generated by the generator-motor.
For example, Patent Literature 1 describes the following method in a hybrid work machine. The hybrid work machine includes: a hydraulic pump; an engine, which is coupled to allow transmission of power to the hydraulic pump; an electric motor, which is coupled to allow driving the hydraulic pump independently from the engine; and a battery. The method detects a variable corresponding to the output state of the hybrid work machine, calculates the average value of the variable during a predetermined time, which is set in advance, and sets the magnitude of the output of the engine corresponding to the calculated average value. This method controls the engine output corresponding to the parameter of the averaged output state of the hybrid work machine. This allows gradually changing the engine output so as to stabilize the operating state of the engine.